Insulated Conductor With A Strippable Layer

Abstract

A composite of polymeric materials which are adheringly joined to each other and which can be easily and cleanly separated by stripping apart with a low pulling force whereupon the contacting surfaces of their interface separate cleanly without retention of any residue from the other, and which comprises the combination of a body of ethylene polymer adjoined to a body of an elastomeric blend of polymers comprising ethylene-propylene rubber admixed with a chlorine containing rubber. The combination of materials is especially advantageous when used in wire and cable constructions as a composite of an electrical insulation and an overlying strippable semiconductive layer.

Description

BACKGROUND OF THE INVENTION

A common type of construction for electrical wires or cables designed for medium to high voltage applications, for example about 15 to 35 KV, as well as other classes of electrical service, comprises combinations of one or more insulating layers and semiconductive layers. In a typical cable structure, for instance, the metallic conductor may be provided with an organic polymeric insulation such as crosslinked polyethylene, and an overlying body of semiconducting material comprising an organic polymeric composition which has been rendered electroconductive by the inclusion therein of electrical conductivity imparting agents or fillers such as carbon black. Although these cable constructions may vary in certain elements, and often include an intermediate component disposed between the metallic conductor and the primary body of dielectric insulation such as a layer of separating tape or inner layer of semiconductive material, or are enclosed within protective covering sheaths, all such cable constructions conventionally include therein at least a body of primary insulation surrounding the conductor with an overlying body of semiconducting material in physical contact with the insulation. However, this arrangement of a layer of insulation with a superimposed layer of semiconductive material thereover incurs certain handicaps.

For example, to prevent the occurrence of ionization or corona formation resulting from internal voids or pockets within the cable construction and consequent ultimate breakdown of the insulation, it is necessary to eliminate the presence or possible occurrence of any free space or voids within or resulting from the interface between the adjoining surfaces of the body of the insulation and the body of semiconducting material. U.S. Pat. No. 3,677,849 deals with this problem of intermediate void spaces at the interface of the insulation and semiconductive material by applying a heat treatment to the assembled product to induce a shrinkage of the semiconductive material tightly about the insulation. U.S. Pat. No. 3,259,688 proposes a different solution to this problem comprising a distinctive construction and an irradiation treatment.

Further, the insulation layer and overlying semiconductive layer for electrical cable can be formed concurrently about the wire or metal conductor by means of a continuous simultaneous extrusion process with one extruder, or these layers are formed in sequence employing tandem extruders, and both layers are thereafter cured at the same time in a single operation and unit to minimize manufacturing steps and apparatus. However, the simultaneous curing of both layers together, or even the curing of only one layer alone while it is in a contiguous arrangement with the other, can result in the apparent formation of crosslinking bonds bridging across the interface between the adjoining surfaces of each phase. The occurence of such crosslinking bonds bridging the interface between the surfaces of said phases renders their subsequent separation such as the removal of a portion of the body of semiconductive material from about the insulation by stripping for the purpose of making splices or terminal connections very difficult. Such separation requires the application of great force, and, upon being peeled off, the semiconductive material is prone to leave a substantial residue of its mass firmly adhering to the other surface or insulation. As is known in the art, it is necessary when splicing and treating cable ends that the semiconductive material be cleanly stripped or completely removed from the terminal section of the cable end without any damage or material loss to the underlying surface of the insulation, whereby the separation can require an appreciable amount of added labor time and costs when the semiconductive material is difficult to remove by stripping and/or a residue thereof is retained tenaciously adhering to the surface of the insulation. The difficulties of this aspect of such cable constructions are the subject of U.S. Pat. No. 3,684,821.

SUMMARY OF THE INVENTION

This invention comprises a combination of specific organic polymeric materials, and a composite costruction formed therewith wherein two phases or bodies are adheringly united with each other at their abutting surfaces to provide a substantially continuous and secure union of their contacting surfaces extending over their common interface and thereby effectively obviating the occurrence of intermediate void spaces, while at the same time providing an interface union between the phases which is easily separated with a relatively small pulling force whereupon the components part with clean surfaces each free of any residue from the other.

The invention includes the combination of a first body of ethylene polymer with a second body composed of an elastomeric blend of a minor portion of ethylene-propylene rubbers admixed with a major portion of a chlorine containing elastomer comprising polychloroprene rubber (neoprene), or chlorosulfonated polyethylene rubber (Hypalon). The compositions and their attributes of this combination are uniquely suitable and advantageous for use in the construction of electrical wires and cables in the function of a composite insulation of ethylene polymer with an easily and cleanly strippable semiconductive material superimposed over the insulation when the polymeric material comprising the said elastomeric blends is rendered suitably electroconductive by appropriately filling with a typical electrical conductivity imparting agent or filler such as carbon black dispersed therethrough, or some other electrically conductive particulate material such as silicon carbide, iron, aluminum, etc., in such amounts so as to impart the desired degree of conductivity.

OBJECTS OF THE INVENTION

It is a primary object of this invention to provide polymeric materials that can be joined in a contiguous relationship with their interfacial surfaces adheringly united together so as to eliminate the presence or any occurrence of intermediate void spaces therebetween, and which thereafter can be separated by the application of a low pulling force with the interfacial surfaces of the bodies cleaving cleanly and free of any adhering residual material.

It is also a primary object of this invention to provide electrical conductors or wire with coverings including a combination of bodies or organic polymeric materials comprising a first layer of insulation with a surface thereof adheringly joined to a surface of a second layer which may be of any suitable thickness down to less than about one millimeter, and wherein the second layer of the polymeric material is easily and cleanly strippable from the first layer of insulation with low peeling effort of preferably of about 2 to 16 pounds pulling force per one half inch wide strip of material, leaving the separated surface of each layer intact, and clean and free of any residue.

It is an additional and specific object of this invention to provide an electrical wire or cable having a multilayered covering about a metallic conductor comprising a combination of cured polymeric materials consisting of an insulation and an overlying semiconductive shield which is free of intermediate voids or spaces at the interface of said materials, and wherein the material consisting of the semiconductive shield comprising a polymeric carrier or matrix for particulate conductive filler material dispersed therethrough can be peeled or stripped off the underlying insulation with little effort or pull and it separates or parts cleanly from the surface of the insulation leaving it intact and without adhering material.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 comprises a perspective view of a portion of an insulated conductor having a semiconductive shield thereon; and,

FIG. 2 comprises a cross-sectional view of the insulation and overlying semiconductive layer about a portion of metallic conductor.

DESCRIPTION OF A PREFERRED EMBODIMENT

This invention is hereinafter described in relation to its principal field of application and utility, the construction of electrical wire and cable, although other areas of application are contemplated.

The invention specifically consists of a novel combination of given polymeric materials, or combined bodies composed thereof, which provide unique interfacial characteristics when their contiguous surfaces are adheringly joined together by curing the polymeric material of at least one of the combined bodies. Polymeric materials of the invention comprise for the one phase, a body or unit of ethylene polymer, and for the other phase of the composite, a body or unit of an elastomeric blend consisting of about 20 to 45 parts by weight of ethylene-propylene copolymer or terpolymer rubbers admixed with about 55 to 80 parts by weight of a chlorine containing elastomer of either polychloroprene rubber or chlorosulfonated polyethylene rubber. Accordingly for the purposes of this disclosure and claims, the term copolymers of ethylene and propylene includes terpolymers of such monomers.

The ethylene polymer of one phase of the combined polymeric bodies includes polyethylene, a common and extensively used electrical insulation material for wire and cable, which is cross-link cured to a thermoset state in keeping with the requirements of the invention. Also included are similar compolymers of ethylene and other polymerizable materials, and blends of such polymers and copolymers which are at least predominately composed of ethylene and are known in the art to provide effective cross-link curable electrical insulations. For example, copolymers of ethylene and vinyl acetate and similar copolymers wherein the ethylene content is a majority of more than 50 percent by weight, and preferably at least about 75 percent by weight of ethylene content. The latter class of copolymers of ethylene and blends for electrical insulating materials for wire and cable are disclosed in the above mentioned U.S. Pats. Nos. 3,259,688 and 3,684,821, and other prior art publications.

The particular elastomeric blends comprising the second phase, which when joined with the ethylene polymer phase together produce the distinctive interfacial characteristics and functions of this invention, preferably consists of about 25 to 45 parts by weight of ethylene-propylene copolymer or terpolymer rubber substantially homogeneously admixed or blended with about 55 to 75 parts by weight of polychloroprene, or alternatively about 25 to 40 parts by weight of the ethylene propylene copolymer or terpolymer rubber substantially homogeneously admixed or blended with about 60 to 75 parts by weight of chlorosulfonated polyethylene. The terpolymers of ethylene-propylene include commercially available rubbers produced by the copolymerization of ethylene and propylene together with minor proportioned dienes such as ethylidiene norbornene, and dicyclopentadiene and 1, 4 - hexadiene. The terpolymers of ethylene-propylene with dienes, as is well known in the art, give greater latitude in the available curing systems in relation to the copolymers of only ethylene and propylene. Specifically, the copolymers require a free radical curing mechanism as provided by a peroxide compound, whereas the terpolymers with this additional unsaturated radicals can also be cured with a conventional sulfur-accelerator curing system, as well as with a peroxide free radical system.

For service in electrical applications such as a semiconductive component in cable for medium to high voltage service, the elastomeric blends can be easily rendered electroconductive to any appropriate degree desired by the filling or inclusion therethrough of a suitable amount of an electrical conductivity imparting agent such as about 15 to 75 parts of carbon black or metal particles by weight of the polymeric ingredients according to conventional practices. When aptly rendered electroconductive with a suitable amount of a conductive material, dispersed therethrough, the elastomeric blend can fulfill the required electrical functions of a semiconducting material in electrical cable, and when combined with an ethylene polymer insulation and cured in accordance with this invention, it provides the unique interfacial properties which effectively eliminate the occurrence of intermediate void spaces between the interface surfaces of insulation and semiconductive materials and also enables an easy and clean separation of the semiconductive material from the insulation.

The organic polymeric materials of each phase of the combination of this invention, both ethylene polymers and the elastomeric blends, are typically cured to a substantially thermoset condition by cross-linking with a peroxide forming free radical according to conventional practices such as described in U.S. Pats. Nos. 2,888,424 and 3,079,370, and subsequent relevant prior art. However, other curing systems or means known to the art or prescribed by the polymer manufacturers or suppliers can be applied, such as the use of sulfur based system with terpolymers of ethylene and propylene. In the preferred peroxide induced cross-linking curing system comprising the use of a tertiary peroxide such as a dicumyl peroxide, it is only required that at least one of the polymeric bodies or phases, either the ethylene polymer or the elastomeric blends, undergoes curing while the surface thereof is in intimate physical contact with the surface of the other polymeric body or phase whereby the curing mechanism of one phase can effect the apparent cross-linking bonds bridging the surfaces to adheringly unite the contacting surfaces of the interface. However, as a practical matter the most expedient manufacturing systems such as the sequential or tandem extrusion of the dual layers of ethylene polymer and overlying elastomeric blends upon the wire core followed by simultaneous curing of both phases, together, would incur the preferred curing of each polymeric phase or material of the combination at the same time to achieve the optimum effects thereof.

Referring to the drawing, a typical cable of medium to high voltage capacity of the type to which this invention is especially applicable and advantageous, is shown in perspective in FIG. 1, and a short portion of such a cable is also shown with the insulation and semiconductive layer in longitudinal cross section about the conductor in FIG. 2. The overall cable product 10, primarily comprises a metallic conductor 12, a relatively thick first body of insulation 14 surrounding the conductor, and overlying the insulation is a second body or layer of semiconductive material 16. Other components can be included in the cable structure following known designs, for example separating paper or tape, or a semiconductive layer located between the metallic conductor 12 and the primary insulation 14, such as shown in the aforementioned Pats. Nos. 3,259,688 and 3,684,821, and the means of this invention apply thereto with its attendant advantages whenever the insulation abuts the semiconductive component as is conventional in medium to high voltage capacity cables. Upon curing at least one component of the superimposed combination, either the body of ethylene polymer insulation 14 or the body of the filled semiconductive material 16, and preferably both together, the insulation and semiconductive material covering the insulation become adheringly joined to each other producing a united interface 18 of unique attributes which eliminates intermediate voids, and upon the application of a small pulling force of only a few pounds the surfaces at the interface separate cleanly leaving each surface free of adherents from the other.

The following comprise specific examples of suitable and preferred polymeric materials for the application of this invention in the construction of high voltage cable comprising a body of polyethylene insulation combined with an overlying body of semiconductive material of a polymeric carrier or matrix comprising an elastomeric blend filled with particulate conductive material.

The ethylene polymer composition comprising the insulation, or one phase or polymeric body of the combination of this invention, consisted of the following typical commercial insulating formula:

EXAMPLE A

Percent Parts by by weight weight Polyethylene, low density -- R-4 Sinclair Koppers Company 62.70 100.00 Calcined Clay -- Whitetex Clay 31.04 50.00 Titanium Dioxide pigment Titanox RA-NC 3.10 5.00 Antioxidant -- Monsanto Flectol-H, polytrimethyldihydroquinoline 1.09 1.75 Vinyl silane 0.93 1.50 Curing agent -- Hercules Di Cup T, di-.alpha.-cumyl peroxide 1.77 2.85

These ingredients were compounded in a suitable mixer, a roll mill, until substantially homogeneously dispersed. However pursuant to conventional practices, all ingredients except for the perioxide were first admixed at elevated temperatures of about 250.degree.F, or within a range of about 200 to 300.degree.F, to flux to polymer and expedite the mixing. Thereafter the mix was cooled to below the decomposition temperature of the particular peroxide curing agent, in this case down to below about 220.degree.F, whereupon the peroxide curing agent was added and dispersed through the mix. The compound was then ready for forming to a given shape and curing by the application of heat.

The following comprises examples of the elastomeric blends comprising ethylene-propylene rubber admixed with chlorine containing elastomers consisting of polychloroprene, which as a body or layer in combination with a body or layer of an ethylene polymer, produces the unique interface characteristics of this invention. In these examples the elastomeric blends were filled with an electrically conductive carbon black so as to perform as a semiconductive material in an electrical cable in combination with a polyethylene insulation of the above formulation.

EXAMPLES I - V

In the following examples, samples composed of the polyethylene composition given in Example A, and a sample of each elastomeric blend formulation given in Examples 1, 2 and 5 hereinafter, were individually sheeted on a hot mill, and a warm strip, measuring about 0.060 to 0.075 inch thickness, of the polyethylene composition was combined with a similar warm strip of each one of the formulations of Examples 1, 2 and 5 of about the same thickness. All three of the thus formed combined strip specimens comprising composite Examples A-1, A-2 and A-5 were each individually molded as composite slabs in a press and cured at 310.degree.F for about 45 minutes to simulate a sequential extrusion molding of one warm layer upon the other followed by a simultaneous curing.

Upon cooling each specimen to room temperature and conditioning each at ambient conditions for approximately 16 hours, a 4 inch long and one-half inch wide section of each composite cured specimen was tested in a Scott tester for strippability, and the pulling force in pounds required to separate the adhering layers of each specimen is given in the following table for Examples I - V.

The formulations of elastomeric blends given in Examples 3 and 4, were respectively extruded in a thickness of about 0.030 inch over an uncured polyethylene insulation of the composition of Example A which had been formed with an extruder around a core of a No. 10 AWG wire conductor in a thickness of about 0.150 inch. Each of said wire specimens of the composits of polyethylene and elastomeric blends were then cured with steam at a temperature of about 406.degree.F for a dwell period of about 2 minutes. After cooling and conditioning at room temperature the pull required for stripping or separating the layer of each sample of polymer composite and its parting characteristics were determined. The pulling force to strip a one-half inch wide section of each of the elastomeric blends of the formulation given in Examples 3 and 4 from the adhesively joined polyethylene composition of Example A is also given in the following Table for Examples 1 - 5. Also each of the specimens were found to separate clean and free of any residue. ##SPC1##

The following comprise examples of the elastomeric blends comprising ethylene-propylene rubber copolymers admixed with a chlorine containing elastomer consisting of chlorosulfonated polyethylene, which as a body or layer in combination with a body or layer of ethylene polymer, also produces the unique interface characteristic of the invention. Example 6 illustrates a ratio of 35 parts by weight of the ethylene-propylene terpolymer to 65 parts of chlorosulfonated polyethylene, and Example 7 is a ratio of 30 parts of ethylene-propylene terpolymer to 70 parts of chlorosulfonated polyethylene. The chlorosulfonated polyethylene rubber was a typical commercial Hypalon rubber designated 40S, with a chlorine content of about 35 percent by weight and a sulfur content of about 1 percent by weight. However, Hypalon or other chlorosulfonated rubber containing from about 20 to 43 percent by weight of chlorine about 1 to 2 percent by weight of sulfur are suitable.

EXAMPLES VI VII Percent Parts Percent Parts by by weight weight Chlorosulfonated polyethylene -- du Pont Hypalon 40S 34.35 65 37.0 70 Ethylene propylene terpolymer -- du Pont Nordel 1320 18.5 35 15.85 30 Conductive carbon black -- Vulcan XC-72 23.8 45 23.8 45 Hydrocarbon oil -- Circosol 4240 oil 8.99 17 8.99 17 Fumed litharge -- TLD-90 (90% fumed litharge dispersed in EPDM) 10.58 20 10.58 20 Crystalline hydrocarbon wax -- Sunoco Anti-Chek 1.06 2 1.06 2 Antioxidant-Agerite Resin D, polymerized 1, 2-dihydro- 2, 2, 4trimethylquinoline 0.26 0.5 0.26 0.5 Trimethylolpropane trimethyacrylate -- SR-350 1.40 2 1.40 2 Curing Agent -- Hercules Di Cup T, di- .alpha.-cumyl peroxide 1.06 2.64 1.06 2.64 100.00 100.00

The semiconductive materials comprising the filled elastomeric blends of the formulation of Examples 6 and were each sequentially extruded, at a rate of about 15 feet per minute, over a polyethylene insulation of the composition of Example A which has been extrusion molded about a No. 2 AWG bare wire having a thin layer (0.006 inch) of semiconductive tape thereabout. The extrusion of the insulation and semiconductive material of each example were carried out in two sequential passes through an extruding apparatus with the polyethylene insulation first formed in a thickness of about 0.160 inch about the tape covered wire core followed by the extrusion of the overlying layer of semiconductive material in a thickness of about 0.035 inch. The polymeric composite of each specimen was then simultaneously cured with steam at a temperature of about 406.degree.F (approximately 250 psig) for a dwell period of about 2 minutes.

The peeling or stripping characteristic for the separation of the layer of semiconductive material from the underlying polyethylene insulation for each specimen was next evaluated. The pulling force required to strip a one-half inch wide section of the 0.035 inch thick semiconductive material from the insulation substratum was measured as 7.3 pounds for the formulation of Example 6 and 6.3 pounds for Example 7. The separation in each case was clean and free of any residue.

Claims

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An easily and cleanly strippable composite of cured polymeric materials comprising a body of an ethylene polymer with a surface adheringly joined to a contacting surface of a body comprising an elastomeric blend of about 20 to 45 parts by weight of a rubbery polymer of ethylene-propylene admixed with about 55 to 80 parts by weight of at least one chlorine containing elastomer selected from the group consisting of polychloroprene and chlorosulfonated polyethylene, said contacting surfaces of polymeric materials being adheringly joined to each other by means of at least one of said polymeric materials having been cured while the said surfaces of each of the bodies are in adjoining physical contact with each other.

2. The easily and cleanly strippable composite of cured polymeric materials of claim 1, wherein said elastomeric blend comprises about 25 to 45 parts by weight of a rubbery polymer of ethylene-propylene admixed with about 55 to 75 parts by weight of polychloroprene.

3. The easily and cleanly strippable composite of cured polymeric materials of claim 1, wherein said elastomeric blend comprises about 30 to 40 parts by weight of a rubbery polymer of ethylene-propylene substantially homogeneously admixed with about 60 to 70 parts by weight of polychloroprene.

4. The easily and cleanly strippable composite of cured polymeric materials of claim 1, wherein said elastomeric blend comprises about 25 to 40 parts by weight of rubbery polymers of ethylene-propylene admixed with about 60 to 75 parts by weight of chlorosulfonated polyethylene.

5. The easily and cleanly strippable composite of cured polymeric materials of claim 1, wherein said elastomeric blend comprises about 30 to 35 parts by weight of rubbery polymer of ethylene-propylene substantially homogeneously admixed with about 65 to 70 parts by weight of chlorosulfonated polyethylene.

6. An insulated metallic electrical conductor having a covering thereon comprising polymeric materials including a composite of an electrically insulating body of cured ethylene polymer with a surface adheringly joined to a contacting surface of an easily and cleanly strippable overlying semiconductive body comprising an elastomeric blend of about 20 to 45 parts by weight of rubbery polymers of ethylene-propylene admixed with about 55 to 80 parts by weight of at least one chlorine containing elastomers selected from the group consisting of polychloroprene and chlorosulfonated polyethylene, said contacting surfaces of the insulating body and overlying semiconductive body being adheringly joined to each other by means of at least one of said polymeric materials having been cured while the said surfaces of each of the bodies are in adjoining physical contact with each other.

7. The insulating metallic electrical conductor of claim 6, wherein the said elastomeric blend contains an electrically conductive filler dispersed therethrough.

8. The insulated metallic electrical conductor of claim 7, wherein the said electrically conductive filler is present in an amount of about 15 to 75 percent by weight of the elastomeric blend.

9. The insulated electrical conductor of claim 6, wherein said elastomeric blend comprises about 25 to 45 parts by weight of a rubbery polymer of ethylene-propylene admixed with about 55 to 75 parts by weight of polychloroprene.

10. The insulated metallic electrical conductor of claim 6, wherein said elastomeric blend comprises about 30 to 40 parts by weight of a rubbery polymer of ethylene-propylene substantially homogeneously admixed with about 60 to 70 parts by weight of polychloroprene.

11. The insulated metallic electrical conductor of claim 6, wherein said elastomeric blend comprises about 25 to 40 parts by weight of a rubbery polymer of ethylene-propylene admixed with about 60 to 75 parts by weight of chlorosulfonated polyethylene.

12. The insulated metallic electrical conductor of claim 6, wherein said elastomeric blend comprises about 30 to 35 parts by weight of a rubbery polymer of ethylene-propylene substantially homogeneously admixed with about 60 to 75 parts by weight of chlorosulfonated polyethylene.